On February 28, 2006, NASA
and the ESA
released a very detailed image of the Pinwheel Galaxy, which was the largest and
most detailed image of a galaxy by Hubble Space Telescope at the time. The
image was composed from 51 individual exposures, plus some extra ground-based
photos.

M101 is a relatively large galaxy compared to the Milky Way. With a
diameter of 170,000 light-years it is seventy percent larger than the Milky Way.
It has a disk mass on the order of 100 billion solar masses, along with a small
bulge of about 3 billion solar masses.

Another remarkable property of this galaxy is its huge and extremely bright
H II regions, of which a total of about
3,000 can be seen on photographs. H II regions usually accompany the enormous
clouds of high density molecular hydrogen gas contracting under their own
gravitational force where stars form. H II regions are ionized by
large numbers of extremely bright and hot young stars.

On photographs M101 can be seen to be asymmetrical on one side. It is thought
that in the recent past (speaking in galactic terms) M101 underwent a near
collision with another galaxy and the associated gravitational
tidal forces caused the asymmetry. In addition, this encounter also
amplified the density waves in the spiral arms of M101. The amplification of
these waves leads to the compression of the
interstellar hydrogen gas, which then
triggers strong star formation activity.